Monday, 7 August 2017

Generating self-signed HTTPS certificates with subjectAltNames

We provide online services via a bunch of different websites, using federated authentication so that if you sign in to our authentication server, you get a * cookie that’s sent to any other server on our domain.

We use local wildcard DNS, so there’s a * record that resolves everything to, and for each developer machine we create a  * record that’s an alias for hostname, so you can browse to<machine> to see code running on another developer’s workstation, or to view your own local development code.

This works pretty well, but getting a local development system set up involves running local versions of several different apps – and since Google Chrome now throws a security error for any HTTPS site whose certificate doesn’t include a “subject alternative name” field, getting a bunch of local sites all happily sharing the same cookies over HTTPS proved a bit fiddly.

So… here’s a batch file that will spit out a bunch of very useful certificates, adapted from this post on

How it works

  1. Get openssl.exe working - I use the version that's shipped with Cygwin, installed into C:\Windows\Cygwin64\bin\ and added to my system path.
  2. Run makecert.bat. If you don't want to specify a password, just provide a blank one (press Enter). This will spit out three files:
    • local_and_hostname.crt
    • local_and_hostname.key
    • local_and_hostname.pfx
  3. Double-click the local_and_hostname.crt file, click "Install Certificate", and use the Certificate Import Wizard to import it. Choose "Local Machine" as the Store Location, and "Trusted Root Certification Authorities" as the Certificate Store.
  4. Open IIS, select your machine, open "Server Certificates" from the IIS snapin, click "Import..." in the Actions panel
  5. Select the local_and_hostname.pfx certificate created by the batch file. If you used a password when exporting your PKCS12 (.pfx) file, you'll need to provide it here
  6. Finally, set up your IIS HTTPS bindings to use your new certificate.

Yay! Security!